Safety feature for control systems



1941- w. J. MQGOLDRICK 3 ,9 3

SAFETY FEATURE FOR CONTROL SYSTEMS Filed Jan. 28, 1959 finmtot William oJlM 'doldxiak Patented F eb. 25, 1941 UNITED STATES SAFETY FEATURE FOR CONTROL SYSTEMS William J. McGoldrick, Minneapolis, Minn assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn, a corporation of Delaware Application January 28,1939, Serial No. 253,309

6 Claims.

This invention is directed broadly to automatic control systems for fuel burners. It is concerned moreparticularly with theaddition of a safety feature to the system disclosed in copending application Serial No. 253,337 filed January 28, 1939 in the name of John M. Wilson.

The main purpose of the invention is to provide a relatively simple control system for fuel burners in which no electromagnetic relays are used but which gives protection against failure of the fuel to ignite, flame failure after ignition has been obtain-ed, and power failure. This protection is obtained by means of a combustion switch which closes when hot, and the present system is designed to preventa start if the combus tions'witch should happen to stick in the hot position.

Other objects will readily become apparent as the following specification is read in the light of which diagrammatically illustrates a control system embodying my invention.

Indicated generally at H is a room thermostat comprising a bimetallic coil l2 and a switch arm I3 connected thereto at its free end and adapted to make contact with the stationary contact H on a decrease in' temperature to a predetermined point. Although a" room thermostat has been specifically disclosed, it will be apparent that this thermostat may be replaced by a boiler thermostat or pressure responsive device or any other type of control instrument desired.

Indicated generally at I5 is a safety switch comprising a pair of resilient contact blades l6 and I1 which are connected at one end to the brackets l8 and I9, respectively. These contact blades are biased downwardly for a purpose to be described later. The blade l'l carries an insulating block l8 which is adapted to cooperate with the upper end of the bimetallic timer I 9 which is fixed at its lower end to the bracket 20. With the timer in the position indicated in the drawing, the upper end engages the block l8 and supports the blade I! in such a manner that its contact 23 is held in engagement with the contact 24 carried by the contact blade l6.

Slidably supported in the plate 28 is a reset member 29 having two insulating members 30 and 3|. Extending through the plate 28 is a stem 32 having a button 33 for the pu pose of providing a means 1'61- 'manually raising the reset member .of the purpose of resetting the safety switch. f

Mounted on resilient blade 35 is a movable contact 36 which is adapted to cooperate with contact 31 carried by the free end of the bimetal timer l9. Closely associated with the timer I9 is an electric heating element 40 which on heating causes the timer to flex toward the left in a direction to cause engagement of the two contacts the' accompanying drawing, the single figure of 36 and 31. Indicated generally at 4| is a combustion switch which comprises a switch arm 42 and a stationary contact 43. This combustion switch is provided with the usual slip friction mechanism so that the contacts will be closed when the thermal element of the switch (not shown) responds to an increase in temperature and open when the thermal element responds to a decrease in temperature. Due to the slip friction mechanism, the switch is responsive to 10 -a change in the direction of the temperature variation rather than to any predetermined temperature.

Pivoted at 50 by means of the arm 5| is a mercury switch 52 which comprises one pair of electrodes 53 and 54 and a second pair of electrodes 55 and 56 and a globule of mercury 5'! which alternately closes the circuit between one pair of electrodes and the other pair ofelectrodes as the switch is tipped back and forth 20 from one position to the others The arm 5| is pivotally connected to a link 58 which in turn is pivotally connected to the bimetal timer l9 so that as the timer flexes in response to the action of the heater 4!] it will tip the mercury switch 52. 25

' The burner motor is diagrammatically illustrated at 60 and the ignition means at 6|. It will be understood that the purpose of the burner motor is to deliver the fuel in the proper form to the combustion chamber where it is ignited by means of the ignition means 6|. It will be noted that the burnenmotor and ignition means are connected in parallel so that the system is a constant ignition system.

Operation In considering the operation of this system, assume that the room thermostat H is satisfied and has been satisfied for a sufllcient length of time for the bimetal timer l9 to have returned to its cold position in which it is straight up and down as shown in the drawing. The normally closed safety switch will be in the position shown, the combustion switch will be in open position due to the fact that the last temperature change in the furnace was a decrease in temperature due to the extinguishment of the fire. The mercury switch 52 will be tipped to its counter-clockwise position at which time the circuit between the electrodes 53 and 54 will be open thereby deenergizing the burner motor and the ignition means and the globule of mercury 51 will bridge the electrodes 55 and 56.

Due to .the fact that the burner motor is deenergized, the temperature in the room will begin to fall and eventually the bimetal element I2 will bring the switch arm l3 into engagement with its stationary contact ll. This action will establish the following circuit energizing the heater 40 for the bimetal timer l9: supply line 55, conductor 5 66, stationary contact I4, switch arm I3, bimetal element I2, conductor 61, bracket I3, resilient contact blade I6, contacts 24 and 23, contact blade I1, bracket I3, conductor 63, conductor 69, heater 46, protective resistance 10, and conductor H which is connected to the other supply line 1-2. Energization of the heater 46 will cause the bimetal element I3 to flex toward the left which will tip the mercury switch 52 in a clockwise direction causing .the mercury 51 to leave the electrodes 56 and 56 and bridge the electrodes 53 and 54. This will establish the following circuit to the burner motor and ignition means: supply line 65, conductor 66, room thermostat II, conductor 6-1, safety switch I5, conductor 68, conductor 15, electrode 53, mercury .51, electrode 54, conductor 16, conductor 11, burner motor 63, and conductors 18 and 19 back to the other supply line 12.,

The conductors 80 and 3| connect the ignition means 6I in parallel with the motor 60 from conductor 16 to conductor 13.

Energization of the burner motor and ignition means will normally produce combustion within the combustion chamber of the furnace which will raise the temperature so that the thermal element of the combustion responsive switch 4| will move the switch arm 42 into engagement with the stationary contact 43. This will normally occur before the bimetal timer I3 has flexed far enough to the left to bring the contacts 36 and 31 into engagement. After the timer has brought contacts 36 and 31 into engagement, the following shunt circuit is set up around the heater 4!]: from the upper end of heater 40, conductor 33, switch arm 42, stationary contact 43, conductor 34, resilient contact blade 35, contacts 36 and 31, bimetal timer I3 and bracket 20 to which the lower end of heater is connected.

Closure of this shunt circuit deenergizes the heater 40 and the bimetal timer will immediately start to cool which will cause it to flex to the right breaking the shunt circuit at the contwin 36 and31. As the shunt circuit is broken, the heater 40 will be reenergized which will again cause the bimetal .timer to heat and reengage thecontacts 36 and 31. This action will continue as long as the room thermostat Ill maintains'the circuit closed at I 3-+'I4 and as long as the combustion responsive switch 4| maintains the switch 42-43 in closed position. While the bimetal timer I3 is oscillating back and forth making and breaking contacts 36 and 31, the mercury switch 52 is also being oscillated, but at this time it is tipped far enough so that it maintains the circuit closed between the electrodes 53 and 54 and hence maintains the burner motor 60 and the ignition means 6| energized. This is the normal running position of the system.

After a time the temperature in the room will increase to the point .where the bimetal element I2 will cause the switch arm I3 to disengage from the stationary contact I4 and this will break the circuit between the control system and the supply conductor 65 and the burner motor and i8- nition means will therefore be immediately shut down, the heater 40 will be deenergized and the bimetal timer I 3 will return to its cold position as shown in the drawing.

these conditions, the combustion responsive switch will not close at 42-43 so that when the bimetal timer I3 has flexed to the left and closed the contacts 36 and 31 no shunt circuit will be set up around the heater 40. The timer I3 will therefore'continue to flex to the left pressing back the contact blade 35 until the free end of the timer I3 slips from beneath the insulating block [6 carried by the resilient contact blade I1. At this time the blade I1 will flex downwardly due to its bias thus separating the contacts 23 and 24 and breaking the circuit to the burner motor and ignition means and also the heater 40. It will be noted that the insulating member 30 on the reset 29 prevents blade I6 from following the blade I1 and maintaining the contacts 23 and 24 closed. The system is now shut down and the bimetal element is cooling but it is prevented from flexing toward the right due to the fact that the insulating block I3 on the blade I1 .is in its path of movement thereby looking it in its flexed position.

The system will remain in this condition until the reset member 29 is manually raised by pushing upwardly on the push button 33. This action will first raise the blade I6 out of the path of movement of the blade I1 and then raise the blade- I1 until-the insulating block I8 frees the bimetal timer I6 permitting it to return to its cold position. When the button 33 is released, the blade I1 will be left in the position shown in the drawing and the blade I6 will return the contact 24 into engagement with the contact 23. The reset operation is arranged in this manner so that theswitch contacts 23 and 24 cannot be manually held in closed position. In other words, the switch is trip free. The system has now been returned to theposition shown in the drawing and another attempt to start will be made if the room thermostat is still calling for heat.

Now assume that after the room thermostat has become satisfied and has shut down the system after a normal run, the contacts 42 and 43 remain stuck in closed position. It will be appreciated that under these conditions, the system if lowed to start will not be protected against a allure to obtain combustion and raw fuel will continue to be pumped into the combustion chamber under these cirrcumstances. In

order to prevent the system from being started under these conditions, the electrodes 55 and 56' have been provided in the mercury switch 52.- When the bimetal timer I3 is in its cold position,

the mercury 51 bridges the electrodes 55 and 56 and the following shunt-circuit is set up around the heater 40 when the room thermostat calls forheat: from the upper end of heater 40, conductor 83, switch arm 42, contact 43, conductor 84, conductor 86, electrode 55, mercury 51, electrode 56, and conductor 81 back to the lower end of heater 40. It will therefore be seen that if the contacts 42 and 43 are stuck in closed position showing the combustion responsive control to be inoperative that there will be a shunt circuit closedabout the heater 46 and the heater will therefore not be energized to cause the bimetal timer I3 to close the circuit to the burner motor and ignition means.

It will therefore be seen that I have provided a control system for a fuel burner using no electromagnetic relays but which will protect against failure to obtain combustion and failure of combustion and also against the danger resulting from the combustion control switch becoming inoperative.

It will be appreciated that the above system has been shown merely for the purposes of illustration and that I intend to be limited only by the scope of the appended claims.

f claim as my inventionr,

1. In a fuel control system, in combination, an electrical fuel controller, a circuit therefor, a fuel controller switch in said circuit, electrical means which when energized closes said fuel controller switch and thereafter renders said fuel controller inoperative, a main switch for energizing said electrical means, circuit connections including a closed but combustion responsive switch to prevent said fuel controller being rendered inoperative by said electrical means if combustion is established, and means to prevent closure of said fuel controller switch by closing said main switch as long as said combustion switch is closed, said means including a circuit shunting at least a portion of said electrical means, said circuit having therein said combustion switch and a switch that is closed when said fuel controller switch is open and open when said fuel controller switch is closed.

2. In a fuel supply control system comprising in combination, fuel supplying means, means for controlling said fuel supplying means comprising a timer, said timer having a first position in which said fuel supplying means is rendered inoperative, a second position in which said fuel supplying means is rendered inoperative, and an intermediate range between said two positions in which said fuel supplying means is rendered operative, electrical means, a condition responsive device for energizing said electrical means to move said timer slowly from said first position toward said second position, means for preventing said timer from moving into said second position comprising a combustion responsive switch having hot and cold positions, and a circuit controlled by said timer for preventing the energization of said electrical means by said condition responsive device when said timer is in the first position and said combustion responsive switch is in its hot position.

3. In a fuel supply control system comprising in combination, fuel supplying means, a timer, switch means in control of said fuel supplying means operated by said timer, said timer having a first position in which said switch means maintains said fuel supplying means deenergized, a

normally closed safety switch also in control of said fuel supplying means, said timer having a second position in which it opens said safety switch to deenergize said fuel supplying means, said timer also having an intermediate range in which said switch means maintains said fuel supplying means energized, a condition responsive device for energizing said timer to start it moving from its first position toward its second position, means preventing said timer from assuming said second position comprising a closedwhen-hot combustion responsive switch; and means controlled by said switch means for preventing said condition responsive device from energizing said timer when said timer is in its first position and said combustion responsive switch is in its hot position.

4. In a fuel supply control system comprising in combination, fuel supplying means, a circuit therefor, a circuit control means and a normally closed safety switch in said circuit, a thermal timer operatively connected to said circuit control means, a heater for said timer, a circuit for said heater, a condition responsive device in control of said heater circuit, said thermal timer having a cold position in which said circuit control means holds said fuel supplying means deenergized, an intermediate range in which said circuit control means holds said fuel supplying means energized, and a hot position in which said timer trips said safety switch to open position, a shunt circuit for said heater including a closedwhen-hot combustion switch, a second switch completing said shunt circuit closed by said timer when it is in its intermediate range whereby said heater is alternately energized and deenergized as said timer alternately opens and closes said second switch, a second shunt circuit for said heater including said closed-when-hot combustion switch and a second circuit control means, said thermal timer being operatively connected to said second circuit control means to close it when said timer is in its cold position whereby said heater cannot be energized by said condition responsive device if said closed-whenhot combustion switch is in its closed position.

5. In a fuel supply control system comprising in combination, fuel supplying means, a circuit therefor, a mercury switch comprising a first pair of electrodes, a second pair of electrodes, and a globule of mercury for selectively bridging said electrodes, said first pair of electrodes being in said fuel supplying means circuit, a normally closed safety switch in said circuit, a thermal timer operatively connected to said mercury switch, a heater for said timer, a circuit for said heater, a. condition responsive device in control of said heater circuit, said thermal timer having a cold position in which the mercury bridges said second pair of electrodes and not said first pair, an intermediate range in which said mercury bridges said first pair of electrodes and not said second pair, and a hot position in which said timer trips'said safety switch to open position,

.a shunt circuit for said heater including a closedwhen-hot combustion switch, a second switch completing said shunt circuit closed by said timer when it is in its intermediate range whereby said heater is alternately energized and deenergized as said timer alternately opens and closes said second switch, a second shunt circuit for said heater including said closed-when-hot combustion switch and said second pair of electrodes, whereby said condition responsive device cannot energize said heater when said timer is in its cold position and said combustion switch is in its hot position.

, 6. In a fuel control system in combination, an electrical fuel controller, a circuit therefor, a fuel controller switch in said circuit, electrical means which when energized closes said fuel controller switch and thereafter renders said fuel. con troller inoperative, a main switch for energizing said electrical means, circuit connections including a switch which is closed in response to the establishment of combustion for limiting the electrical energy delivered to said electrical means to an amount sumcient to maintain said fuel controller switch closed but not suflicient to render said fuel controller inoperative, and means to prevent closure of said fuel controller switch as long as said combustion switch is closed.

, WILLIAM J. McGOLDRICK. 

